Convolvulaceae) from India with Implications for an East Gondwana Origin of Convolvulaceae
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Paleocene Ipomoea (Convolvulaceae) from India with implications for an East Gondwana origin of Convolvulaceae Gaurav Srivastavaa, Rakesh C. Mehrotraa, and David L. Dilcherb,1 aBirbal Sahni Institute of Palaeosciences, Lucknow 226 007, India; and bDepartment of Geology, Indiana University, Bloomington, IN 47405 Contributed by David L. Dilcher, April 10, 2018 (sent for review January 17, 2018; reviewed by Nina Lucille Baghai-Riding and Gregory Retallack) The morning glory family, Convolvulaceae, is globally important in Materials and Methods medicine and food crops. The family has worldwide distribution in A collection of 200 angiosperm leaves has been made from the Tura For- a variety of habitats; however, its fossil record is very poorly mation exposed in Nangwalbibra (25° 26′ 47.4″ N; 90° 42′ 28.9″ E), East Garo documented. The current fossil record suggests an origin in North Hills District, Meghalaya (Fig. 1 A and B). Here we describe a fossil leaf America, which is in contrast to molecular data that indicate an morphotype based upon 17 specimens, 2 of which have counterparts. The East Gondwana origin. We report Ipomoea leaves from the late specimens are impressions/compressions. They were prepared with the help Paleocene (Thanetian; 58.7–55.8 million years ago) of India, which of a fine chisel and hammer and then photographed in natural low-angled was a part of East Gondwana during this time. This is the earliest light using a 10-megapixel digital camera (Canon SX110). An attempt was fossil record for both the family Convolvulaceae and the order made to extract cuticle with no success. The terminology used in describing Solanales. This suggests that the sister families Convolvulaceae the fossil leaves is based on Dilcher (20) and Ellis et al. (21). The distance between adjacent secondary veins was measured, and a range (minimum– and Solanaceae diverged before the Eocene in Gondwana- maximum) was taken while describing the fossil leaves. The fossils are housed derived continents. The evidence presented here supports the con- in the museum of the Birbal Sahni Institute of Palaeosciences, Lucknow, India. clusion from molecular phylogenetic analysis of an East Gondwana Similar extant leaves were examined at the herbaria of the National Botanical origin of Convolvulaceae. Research Institute, Lucknow, India, and the Forest Research Institute (FRI), EVOLUTION Dehradun, India. In addition, the website of the Royal Botanical Gardens, Kew Convolvulaceae | sweet potato | Ipomoea | Paleocene | East Gondwana (apps.kew.org/herbcat/navigator.do; accessed on August 18, 2014 and January 15, 2017) was also consulted. onvolvulaceae, the morning glory family, consists of 57 gen- Results and Discussion Cera and 1,880 species (1) with a worldwide distribution (SI Appendix, Fig. S1A) over a broad range of habitats. Most Systematics. members of the family are perennial herbs, vines, or woody li- Order. Solanales Juss. ex Bercht. et J. Presl. anas, rarely annual herbs, shrubs, or trees that are endemic to the Family. Convolvulaceae Juss. Genus. Ipomoea L. tropical regions (2), although weedy taxa from temperate regions Species. Ipomoea meghalayensis sp. nov., Srivastava, Mehrotra, are also known (e.g., Calystegia and Convolvulus). Traditionally, and Dilcher. the family was placed in Solanales (3–5), but Takhtajan (6) created Holotype. Specimen no. Birbal Sahni Institute of Palaeosciences a new order, Convolvulales, for it. Recent molecular systematics (BSIP) 40571 (Figs. 2A and 3D). places the family in the order Solanales (7). Ipomoea batatas (sweet ’ Paratypes. Specimen nos. BSIP 40572, 40573A, 40573B, 40574, potato) of the Convolvulaceae family is the world s second most and 40575–40586 (Figs. 2 B–F,3A–C,4A–D, and 5 A–E). important root crop (8). Despite a large number of species having Repository. Birbal Sahni Institute of Palaeosciences, Lucknow. worldwide distribution in varied environmental conditions, the Type locality. Nangwalbibra, Meghalaya, India (Fig. 1). fossil record of the family is very sporadic and is represented by a Horizon. Tura Formation. leaf from late Eocene sediments of the United States (9) and by Age. Late Paleocene (Thanetian; ∼58.7–55.8 Ma). pollen from Miocene sediment of Australia (10). Fossil records of its sister family Solanaceae are known from pollen and seeds from Significance Eocene sediments of Colorado and of Lower Bagshot, United Kingdom (11). However, the recent discovery of Physalis fruit from The origin of the worldwide-distributed morning glory family the early Eocene (52.22 ± 0.22 Ma) sediments of Patagonia raises (Convolvulaceae) has been a matter of debate. The fossil record the possibility of a divergence in Gondwana (12). from late Eocene sediments of North America argues for a Molecular phylogenetic studies suggest that the Convolvulaceae Laurasian origin that is in contrast to a molecular phylogenetic is monophyletic (13) and diverged from Solanaceae around 65 Ma study that favors an East Gondwana origin. This report of fossil (14), 85 Ma (15), 55.3 Ma (69.98–47.80 Ma) (16), or 66 Ma (17) in Ipomoea from the late Paleocene of India supports the current East Gondwana (18). molecular phylogenetic conclusions of an East Gondwana ori- The Eocene fossil records of Convolvulaceae and Solanaceae gin for Convolvulaceae. from Laurasia, together with the Eocene Physalis fruit from Gondwana, raise an important question about the early di- Author contributions: G.S., R.C.M., and D.L.D. designed research; G.S., R.C.M., and D.L.D. vergence of both families and Solanales. Here we report fossil performed research; G.S. contributed new reagents/analytic tools; G.S., R.C.M., and D.L.D. leaves of Ipomoea (Convolvulaceae) from the late Paleocene analyzed data; and G.S. and D.L.D. wrote the paper. (Thanetian; ∼58.7–55.8 Ma) of Meghalaya (25° 26′ 47.4″ N), Reviewers: N.L.B.-R., Delta State University; and G.R., University of Oregon. India (Fig. 1 A and B), which was located ∼7° S paleolatitude The authors declare no conflict of interest. (19) (www.odsn.de)(SI Appendix, Fig. S1D). These fossil leaves Published under the PNAS license. represent the earliest record of Convolvulaceae and also Solanales, 1To whom correspondence should be addressed. Email: [email protected]. and they document the early divergence of Convolvulaceae and This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. Solanaceae supporting the molecular inference of an East Gondwana 1073/pnas.1800626115/-/DCSupplemental. origin of the Convolvulaceae (18). www.pnas.org/cgi/doi/10.1073/pnas.1800626115 PNAS Latest Articles | 1of6 Downloaded by guest on September 26, 2021 Fig. 1. Physiographic map showing the fossil locality. (A) Physiographic map of India and adjoining areas showing the fossil locality (red box). (B) High- resolution physiographic map showing the fossil locality (red asterisk) (map source: www.marine-geo.org/tools/maps_grids.php). Etymology. The specific epithet is named for the locality. show superficial resemblance to our fossil, but the presence of an acute and rounded leaf base, moderately wide acute angle of di- Description. Lamina symmetrical, elliptic to ovate (widest at base vergence of secondary veins, and reticulate tertiary veins differ- or proximal 1/fourth); apex attenuate, arising somewhat abruptly entiate the modern species from the fossil (SI Appendix, Table S1). from a broader leaf blade, rounded at the top (SI Appendix, Fig. A large number of modern species of Ipomoea were compared S2A); base is deeply cordate, recurved leaf lamina joining the in detail with the fossil leaves, and a comparative chart of them is petiole (Figs. 2D and 5 A and B and SI Appendix, Fig. S2D). The presented in SI Appendix, Table S1. Our fossil shows close re- basal lobes are rounded; margin entire, slightly wavy; midvein semblance with the modern leaves of Ipomoea eriocarpa R. Br. in slightly curving, tapering, multistranded, and stout; leaf texture the characters such as attenuate apex, deeply cordate base, pinnate thin, appearing rough; petiole moderate to stout, short (0.03– eucamptodromous venation, narrow-to-moderate angle of diver- 1.78 mm); venation pinnate, eucamptodromous; secondary veins gence of secondary veins, distance between adjacent secondary slightly more acute toward apex, angle of divergence often 57° to veins, presence of intersecondary veins, and percurrent tertiary 67°, sometimes decurrent to midvein (Fig. 3D), course straight and veins (Fig. 6 A–D). The petiole is short in the fossil, while in the arching upward at about two-thirds of the distance to the margin, modern leaf of I. eriocarpa the petiole length varies from short to forming loops close to the margin after joining with superadjacent long (0.6–6.5 cm) (SI Appendix,Fig.S4A and B). There are also secondary veins, two basal secondary veins at broad angles, each several other species of Ipomoea where length of the petiole varies associated with a smaller more basal vein feeding the lobes (SI Ap- from 2 mm to 14 cm (2). Secretory structures have been reported pendix,Fig.S2B); tertiary veins percurrent (Fig. 3D and SI Appendix, in Ipomoea species in the form of glandular trichomes (22–25), Fig. S2C); one or two intersecondary veins may be present (Fig. 3D), laticiferous canals and idioblasts (24, 26), floral nectaries on sepals sometimes joining the midvein with the secondary vein (Fig. 3D); (27), and extrafloral nectarines on petioles (24, 28–31). Glandular glandular dots numerous (Fig. 2C and SI Appendix,Fig.S3A–C). dots